1. Field of the Invention
The present invention relates to optical disc drives for reading information from different kinds of optical discs by using a plurality of beams of different wavelengths. In particular, the present invention relates to onboard optical disc drives for installation in automobiles, for example.
2. Description of the Related Art
Optical disc drives capable of reading information recorded on optical discs are widely used in audio visual devises with which people can enjoy music and movies. Many optical disc drives in recent years are designed to be compatible with different kinds of optical discs including CDs, DVDs and BDs (Blu-ray Disc). FIG. 5 shows such an optical disc drive (see JP-A-2004-134056 Gazette). An optical disc drive X shown in the figure is capable of reading information from three kinds of optical discs, i.e. CDs, DVDs and BDs, and includes a chassis which is movable radially of the optical disc, a red unit 91A, an infrared unit 91B, a blue unit 91C and an objective optical system 92 mounted on the chassis.
The red unit 91A and the infrared unit 91B are hologram units which are capable of emitting and receiving a red beam for reading DVDs and an infrared beam for reading CDs, respectively. The blue unit 91C is capable of emitting and receiving a blue beam for reading BDs, and includes a blue LD 91Ca, a light receiving element 91Cb, a monitor light-receiving element 91Cc and a beam splitter 91Cd. The objective optical system 92 is an optical system to be faced to the recording surface of the optical disc which is supported and rotated by a spindle Sp, and includes two object lenses 92a, 92b disposed in line extending in the radial direction of the optical disc.
A red beam and an infrared beam emitted from the red unit 91A and the infrared unit 91B pass through collimating lenses 93A and 93B, respectively, to become parallel rays and pass through the beam splitter 94 and come to the object lens 92a. Subsequently, the red beam and the infrared beam are condensed by the object lens 92a on the recording surface of the optical disc. The red beam and the infrared beam are then reflected by the recording surface of the optical disc, and then propagate along the optical paths altered appropriately by the beam splitter 94 before being received by the red unit 91A and the infrared unit 91B, respectively. The monitor light-receiving element 95 monitors the output from the red unit 91A and the infrared unit 91B.
A blue beam emitted from the blue LD 91Ca passes through the beam splitter 91Cd, then through a collimating lens 93C to become a parallel ray, and then travels to the object lens 92b. Subsequently, the blue beam is condensed by the object lens 92b on the recording surface of the optical disc. The reflected beam is then directed by the beam splitter 91Ce to the light receiving element 91Cb and the monitor light-receiving element 91Cc. As is clear from the above, according to the optical disc drive X, three kinds of optical discs, i.e. CDs, DVDs and BDs, can be read with one apparatus.
However, the optical disc drive X has the following problems when it is to be incorporated in an automobile: The optical disc drive X has two object lenses 92a, 92b disposed in line extending in the radial direction of the optical disc. Further, the infrared unit 91B is disposed radially outward of the optical disc. For these reasons, the chassis has to be relatively large in the disc's radial direction. In order to read information recorded in the recording surface of the optical disc, each of the two object lenses 92a, 92b is required to face any region from the radially innermost region to the radially outermost region of the recording surface on the optical disc. When reading the radially outermost region of the recording surface, the chassis is moved radially outward of the disc, and it is accordingly unavoidable that part of the chassis comes significantly out of the optical disc. Therefore, the optical disc drive X has to be big in size, and there has been a requirement for further size reduction for installation in automobiles.